<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        //让用户输入月份
        var month = Number(prompt('请输入月份'));
        //分类讨论
        switch (month) {
            case 1:
            case 3:
            case 5:
            case 7:
            case 8:
            case 10:
            case 12:
                alert('这个月有31天');
                break;

            case 4:
            case 6:
            case 9:
            case 11:
                alert('这个月有28天');
                break;

            case 2:
                alert('这个月有28天或者29天');
                break;
            default:
                alert('你输入的月份有误')

        }
    </script>
</body>

</html>